In the case of aircraft doors, as possible closing devices for apertures in the fuselage, which apertures after landing are to be opened manually even if there is a pressure difference between the cabin (as a rule the side with the higher pressure) and the environment (as a rule the side with the lower pressure), there is the danger that the operator of the door is hurt as a result of the quick opening movement of the door or as a result of the ensuing airstream from the cabin to the outside. As proven by accidents, this particular condition (fault) in which overpressure builds up in an aircraft cabin is a potential source of accidents or danger. In order to reduce the likelihood of accidents from this source, in passenger aircraft, in particular in the passenger doors of the Airbus fleet there is an electrically-operated visual warning display in the form of a small light in the window funnel of the door. The function of the visual warning display installed in the door depends on the on-board network.
If there is a differential pressure and at the same time the triggering of the evacuation-slide is deactivated, the visual warning display emits a visual signal which is to indicate to the user that operation of the door represents a potential hazard and should not be attempted until such time as pressure equalisation has taken place. This poses no grounds for concern if the people on board leave the aircraft in the normal way. In the case of an emergency evacuation of the aircraft, in which case all passengers will have to have left the aircraft within a period of 90 seconds, untrained persons, such as the passengers, would not operate the doors because of the blinking warning light. Consequently there would be a serious delay in the evacuation.
Different philosophies exist in relation to the planning of an emergency evacuation.
On the one hand it is held that in the case of an emergency situation the evacuation time should not be delayed by a small potential hazard posed by a pressurised cabin and by an inordinately long period of waiting. In this case, consequently, the display of the hazardous situation should be omitted.
On the other hand it is held that even in an emergency situation the hazard should be displayed because at any rate only experienced and trained specialist personnel operate the doors and know how to interpret the indications. It should therefore be possible to set various prerequisites for a hazard notification.
Furthermore, as a result of the necessary electrical energy supply the warning function of the visual display is limited to scenarios in which a current supply from the on-board network is available. The warning function is therefore only available if an on-board voltage is present, either externally or internally. Since, in addition, the warning function takes place only visually, a dependency on the degree of illumination of the surroundings is given. Diffused light that enters in an unfavourable manner can thus make it very difficult for an operator to see the warning function.
In order to avoid visual disturbance with a warning function, WO 2004/022425A1 discloses a device to warn of differential pressure during opening of a pressurised closing device of an aperture in the fuselage. Thereby an air duct from the side with higher pressure (passenger cabin) to the side with lower pressure (outside environment) is provided. The air duct can be closed by a valve, wherein said valve can be controlled by means of a control lever that is in effective connection with the door opening mechanism.
Disadvantageously this acoustic signal that has been generated in a mechano-acoustic way is poor in the efficiency for the sound generation. The warning sound is made from air, or more precisely, by air streaming through the aperture. The sound pressure of the warning sound achieved thus directly depends on several parameters. Consequently, an audible warning signal is generated only under very specific preconditions. The physical reasons for this are firstly the very small “quantity” of energy available (stored in the air volume that flows through the aperture) and secondly the characteristics associated with noise generation with pipes or by whistling. Not only does the efficiency of less than 1% play a part, but also the fact that little airflow through a pipe does not generate a sound at all, while excessive airflow through the pipe leads to so-called “overblowing” (so that the pipe octaviates).
In both cases (little pressure or a lot of pressure) no unequivocal warning sound is generated. In a mechano-acoustic generation of the warning signal the warning signal is produced by flow, whereby no unequivocal allocation of the signal type is possible. Furthermore, no test function can be implemented without creating real conditions. Due to the warning signal being generated in a purely mechanical way, retrofitting requires intervention in the mechanics so that integration is time-consuming and cost expensive. Furthermore, the mechanism is susceptible to icing, and it is possible that signals can only be generated with low acoustic pressure (“low volume”).
U.S. Pat. No. 5,337,977 discloses a ventilation aperture in an aircraft door, which ventilation aperture shall prevent that the door, which may be pressurised, can be opened. No warning function is generated; instead, the described device blocks the opening procedure if there is overpressure in the cabin. Consequently, any evacuation that may be necessary may be delayed.